Much of the electrical power in use worldwide is distributed at high alternating current (AC) from the location at which it is generated to a location proximate to where is will be utilized. High voltage allows power transmission over long distance with reduced wiring cost and resistive losses. Voltage can then be reduced, usually in stages, to lower voltage suitable for intended loads. This power distribution system is often referred to as the “power grid” or, simply, “grid”.
However, most electrical and electronic devices operate as a substantially fixed voltage, referred to as direct current (DC) and are often arranged to be temporarily connected to the grid or disconnected therefrom during which time they are operated from batteries that may be recharged from the power distributed on the grid. Accordingly, such devices, referred to as “offline”, require power conversion from AC to DC power and often the DC power is regulated at a much lower voltage than the voltage available from a connection to the grid. (More specifically, “offline power supply” is defined as a power supply in which the line voltage is rectified and filtered without using a line frequency isolation transformer, which does not preclude inclusion of a high frequency isolation transformer.) Devices that perform such conversion and possibly voltage regulation are often referred to as “adapters” and may be integrated with the electrical device for which they supply power or constructed as a separate structure with wires and fixtures for connection to both the grid and the electrical device.
At the present time, large numbers of low power adapters are commercially required since a large proportion of currently available consumer electronics product are marketed with an adapter. The predominant power ratings for commercially available adapters has been 90 Watts and 65 Watts but the demand for those ratings has been slightly declining in recent years in favor of adapters rated at 45 Watts, due to reduced power requirements of currently available microprocessors. Nevertheless, demand for 65 and 90 Watt adapters remains very strong.
So-called flyback converters (essentially a buck-boost topology DC-DC converter including a transformer for isolation and so-called because the energy transferred to the secondary side of the transformer is reflected back to the primary side when the primary side is “off” or non-conducting) are widely used in low power adapter applications, particularly for power requirements of 75 Watts or less since there is no power factor correction requirement and the topology is very simple and of low cost. Above 75 Watts, a two stage power converter is used to meet power factor requirements.
The best 90 Watt rating adapter currently on the market in terms of efficiency and power density is comprised of an h-bridge PFC stage and an LLC resonant converter stage. The adapter has a power density of about 11 W/in3 and a peak efficiency of about 93% with a worst case efficiency of about 91.8% at 90 V input. The best commercially available 65 Watt adapter uses a flyback topology power converter and achieves a power density of about 92.2% with a worst case efficiency of about 90.6% at 90 volts input. Both of these adapters operate at a frequency in the range of 100-200 kHz and most of the volume is occupied by passive components. The EMI filter must be large to attenuate the relatively low switching frequency noise and both require hand made magnetic components which are not cost-effective to construct and are very subject to parameter variation. Therefore, while these adapters are considered to be the best adapters commercially available from the standpoint of power density and efficiency, higher efficiency and further reduced volume are desirable, particularly as required power ratings for adapters are being reduced.